I. Field
This invention is related to the field of display devices, such as projection display devices. More particularly, the invention is related to methods and systems for attenuating light when displaying images with such display devices.
II. Description of Related Art
Display technology (e.g., for use in computer and entertainment display devices) continues to advance, as generally is the case with consumer and business electronics. Display devices (such as digital display projectors, flat panel displays, plasma displays, cathode-ray-tube (CRT) displays, etc.) continue to improve in the quality and resolution of the images they display. A wide variety of such a display systems is available from InFocus Corporation of Wilsonville, Oreg., the assignee of the present application.
Projection display devices, such as those manufactured by InFocus, include an optical subsystem for displaying images (e.g., still images or video). Such optical subsystems typically include an illumination source (e.g., a high pressure mercury lamp) for generating light. Light is typically generated in a plurality of primary colors (e.g., red, green and blue) using a color wheel or, as one alternative, using separate light sources for each primary color, where the separate light sources are turned on in sequence. The generated light is then directed, focused and modulated to display images (e.g., still or moving) on a display surface, such as a screen or wall. Light may be modulated using any number of approaches. For example, a digital micro-mirror device may be used to modulate light. Such devices are know and are described in U.S. Pat. No. 6,870,660, entitled “Digital micromirror device having mirror-attached spring tips”, which is incorporated by reference herein in it entirety. As an alternative, a liquid crystal array may be used to modulate the primary color light to generate displayed images. Of course, other approaches are possible.
Such systems typically include service logic that is implemented in software, hardware and/or firmware for analyzing the content of the image or images being displayed the display system. Based on the information produced by this image analysis, the display device may modify the manner in which images are displayed.
As one example, based on analysis of an image or series of images (e.g., video content) being displayed, the display device may determine that the content of the image or images is generally dark. When viewing such video content, it may be observed that the brightness transitions are relatively poor as compared with brighter scenes. Due to limitations in the number of steps of brightness available (e.g., resolution), steps from one level of brightness intensity to the next may be visibly abrupt to a viewer of the displayed images. These abrupt steps negatively affect the quality of the image and, therefore, the viewer experience.
As an example of this situation, for a display device that displays images with 8 bits of resolution there are 256 levels of brightness that are available. If these 256 levels are divided equally over the entire range of possible brightness (e.g., in a stepwise linear fashion), transitions from one level to the next will be more readily observable by a viewer at the lower brightness levels due the percentage change in brightness that occurs during those transitions. For instance, transitions at the lowest brightness levels will result in one-hundred percent change in brightness. Accordingly, these transitions are often readily observable when displaying video content with a low level of brightness (e.g., dark scenes).
One approach that is used to address this concern is to reduce the range of brightness over which the number of brightness steps is spread. This is accomplished by placing a solid object in a portion of the path of the light that is used for projecting the video content. For instance, if it is determined by the display device (e.g., using the image analysis service logic described above) that the desired maximum brightness for a particular portion of the video content (e.g., a dark scene in a movie) is half of the maximum white brightness available, then the solid object may be placed such that it reduces the amount of light used to the project the scene by fifty percent. This results in the reduction of the brightness steps by half as well, as the 256 steps are divided over half of the brightness range (e.g., half of the light is blocked).
While such an approach may reduce the perception of abrupt brightness transitions, such approaches still have certain drawbacks. For example, because the light is completely blocked in half of the field and not blocked in the other half, there may still be portions of the displayed images that have abrupt brightness transitions, such as in the portions of the images corresponding with the unblocked portion of the field. Additionally, other portions of the image may have poor contrast, such as in the portions of the images corresponding with the completely blocked portion of the display field. Based on the foregoing, alternative approaches for displaying images or video content with reduced brightness are desirable.